[wpilib] Add functional interface equivalents to MotorController (#6053)

This does not deprecate any current functionality, but prepares the way for future deprecation.

The drive classes now accept void(double) functions, which makes them more flexible.

The C++ API ended up a bit more verbose, but the Java API is really concise with method references, which is >80% of our userbase. For example:

`DifferentialDrive drive = new DifferentialDrive(m_leftMotor::set, m_rightMotor::set);`

Lambdas can be passed to interoperate with vendor motor controller APIs that don't have e.g., set(double), so CTRE doesn't have to maintain their WPI_ classes anymore.

MotorControllerGroup was replaced with PWMMotorController.addFollower() for PWM motor controllers. Users of CAN motor controllers should use their vendor's follower functionality.

wpilibc/src/test/native/include/motorcontrol/MockMotorController.h

@@ -4,10 +4,14 @@
 
 #pragma once
 
+#include <wpi/deprecated.h>
+
 #include "frc/motorcontrol/MotorController.h"
 
 namespace frc {
 
+WPI_IGNORE_DEPRECATED
+
 class MockMotorController : public MotorController {
  public:
   void Set(double speed) override;
@@ -22,4 +26,6 @@ class MockMotorController : public MotorController {
   bool m_isInverted = false;
 };
 
+WPI_UNIGNORE_DEPRECATED
+
 }  // namespace frc

wpilibc/src/test/native/include/motorcontrol/MockPWMMotorController.h

@@ -0,0 +1,23 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+namespace frc {
+
+class MockPWMMotorController {
+ public:
+  void Set(double speed);
+  double Get() const;
+  void SetInverted(bool isInverted);
+  bool GetInverted() const;
+  void Disable();
+  void StopMotor();
+
+ private:
+  double m_speed = 0.0;
+  bool m_isInverted = false;
+};
+
+}  // namespace frc